This invention relates generally to X-ray apparatus and is concerned more particularly with an X-ray tube having adjustable focusing means.
Generally, in an X-ray tube, of the line focusing type, electrons emitted from a filamentary cathode are beamed toward a spaced anode target surface, which is sloped in the direction of a radially aligned, X-ray transparent window in the tube envelope. The electron beam impinges on an aligned area of the sloped target surface, commonly called the "actual focal spot area," which has a configuration corresponding to the incident cross-section of the beam. Thus, the beamed electrons bombarding the actual focal spot area of the target surface generate a useful X-ray beam which passes through the radially aligned, X-ray transparent window. Accordingly, from the direction of the window, the X-ray beam appears to be emanating from a radial projection of the actual focal spot area on the sloped target surface, commonly called the "projected focal spot."
In order to provide the high resolution required for visualizing fine detailed structure encountered in radiography, it is necessary that the projected focal spot have sufficient resolving power and X-ray intensity. Consequently, in conventional X-ray tubes of the line focusing type, the electron emitting cathode generally is disposed in an equipotential cup having a slotted open end from which electrons are directed toward the sloped target surface of the anode. Electrons emerging from the cup are electrostatically accelerated toward the target surface in a flat beam having a generally rectangular cross-section. The resulting rectangular, actual focal spot area extends longitudinally with the slope of the target surface and, consequently, is disposed at a corresponding angle with the radially aligned, X-ray transparent window. Accordingly, from the direction of the window, a radial projection of the rectangular, actual focal spot area of the sloped target surface forms a small square, projected focal spot from which the X-ray beam appears to originate. Thus, the small square, projected focal spot can be made comparable to a point source of X-radiation by reducing it in size as much as possible. However, such reduction in size is limited by the diagonally disposed corners of the square configuration.
In U.S. Pat. No. 3,743,836 granted to W. P. Holland et. al., and assigned to the assignee of this invention, there is disclosed an X-ray tube of the line focusing type which provides a small circular, projected focal spot having a centralized peak region of maximum X-ray intensity, in the manner of a Gaussian distribution of X-ray energy. The electron emitting cathode is insulatingly supported within a cathode focusing cup which is maintained at a negative potential with respect to the cathode. Electrons emerging from the cup are electrostatically accelerated toward the sloped target surface in a beam having an elliptical cross section. The resulting elliptical, actual focal spot area extends longitudinally with the slope of the target surface and, consequently, is disposed at a corresponding angle with the radially aligned, X-ray transparent window. Accordingly, from the direction of the window, a radial projection of the elliptical, actual focal spot area of the sloped target surface forms a small circular, projected focal spot from which the X-ray beam appears to originate. Thus, the small circular, projected focal spot will more closely approximate a point source of X-radiation the more it is reduced in size.
In both of the described line focusing types of X-ray tubes, the size of the projected focal spot is dependent on the size of the associated actual focal spot area, which may vary with changes in cathode current and anode-cathode voltage. The cathode current, for example, may be adjusted during operation of the tube to obtain greater electron emission for increasing the intensity of the X-ray beam. However, the resulting greater density of electrons in the beam impinging on the actual focal spot area of the target surface produces a proportionate increase in space charge repulsion which causes the electron beam to spread. Consequently, the size of the actual focal spot area expands conformingly and produces an associated increase in the size of the projected focal spot. Thus, the approximation of the projected focal spot to a point source of X-radiation decreases; and the resolution provided by the resulting X-ray beam deteriorates accordingly.
Therefore, it is advantageous to provide an X-ray tube of the line focusing type with means for adjusting the size of the effective focal spot independently of changes in cathode current and anode-cathode voltage.